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United States Patent |
5,054,087
|
Carbon
,   et al.
|
October 1, 1991
|
Process and apparatus for optically checking perforations in hollow
articles such as turbine blades
Abstract
A process for optically checking perforations in a hollow article,
particularly the micro-perforations in the vicinity of the leading or
trailing edge of a hollow turbine blade for a turbo-shaft engine,
comprises illuminating the inner cavity of the article through an opening
at one end thereof, scanning the length of the article by means of a video
camera and making a record of the luminance of the reflected light
received by the camera through the perforations to be checked, converting
the sequence of the data thus collected into electric signals, storing the
said signals in a computing and storage unit, and processing and comparing
the signals with a predetermined train of reference signals derived from a
standard article.
Inventors:
|
Carbon; Vincent A. (Paris, FR);
Meiffren; Jean-Luc C. (St. Germain Les Corbeil, FR);
Pailliotet; Pierre M. (Morsang Sur Orge, FR)
|
Assignee:
|
Societe Nationale d'Etude et de Construction de Moteurs d'Aviation (Paris, FR)
|
Appl. No.:
|
407584 |
Filed:
|
September 15, 1989 |
Foreign Application Priority Data
Current U.S. Class: |
382/152; 356/237.1; 382/256 |
Intern'l Class: |
G01N 021/00 |
Field of Search: |
382/1
380/8
356/378,237,240
364/507
|
References Cited
U.S. Patent Documents
343656 | Apr., 1969 | McCartney | 250/223.
|
3495915 | Feb., 1970 | Watson et al. | 356/167.
|
3680966 | Aug., 1972 | Cofek et al. | 356/241.
|
4484081 | Nov., 1984 | Cornyn, Jr. et al. | 250/563.
|
4555798 | Nov., 1985 | Broadbent, Jr. et al. | 382/8.
|
4687328 | Aug., 1987 | Shiraishi et al. | 356/384.
|
4766325 | Aug., 1988 | Merkenschlager et al. | 250/572.
|
4783751 | Nov., 1988 | Ehrlich et al. | 364/506.
|
4803639 | Feb., 1989 | Steele et al. | 364/507.
|
4901361 | Feb., 1990 | Glenn et al. | 382/18.
|
Other References
Microtecnic, No. 4, 1976, p. 30, Reader Service No. 39, Zurich, CH, "Rolls
Royce Reduce Inspection Time From 3 Hours to 10 Minutes."
|
Primary Examiner: Boudreau; Leo H.
Assistant Examiner: Klocinski; Steven P.
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt
Claims
We claim:
1. A process for optically checking the perforations of a hollow article
having an internal cavity bounded by a wall provided with a plurality of
perforations therethrough, and an opening into said cavity, comprising the
steps of:
using a light source to illuminate the internal cavity of said article
through said opening;
scanning a video camera along the outside of said article to receive light
reflected from within said internal cavity outwardly through said
perforations and thereby collect luminance data relating to said reflected
light transmitted through said perforations;
converting the sequence of said collected luminance data into electrical
signals;
storing said signals in a computing and storage unit; and
processing said signals for comparison with a predetermined train of
reference signals derived from a standard article;
wherein said processing of the signals includes the steps of
binarizing said signals, the area seen by said camera being divided into
pixels of known dimensions;
effecting section by section processing of the grey level intensity curve
I=f(d) where d is the coordinate along the section axis;
performing an erosion-expansion treatment by the addition of an interval
function g(d) where g assumes the values 0 or 1 depending on the length d;
affecting smoothing on a plurality of p points of the curve I'=f(d)+g(d)
thus obtained, the smoothed value
##EQU3##
being reallotted to the mean index point between i and i+p to obtain a
smoothed intensity curve I"(d);
extracting the point of inflection of the curve I"(d); and
reconstructing the skeleton image of each open-perforation.
2. A process according to claim 1, including the step of displaying an
optical signature of said article scanned by said video camera on a video
screen for visual analysis of said optical signature.
3. A process according to claim 1, wherein said article is arranged in such
a manner that the axis of symmetry of said internal cavity is in the
vicinity of the optical axis of said light source which illuminates said
cavity.
4. A process according to claim 1, wherein said article and said light
source are arranged so that the number of intermediate reflections
experienced by said light emitted by said light source into said internal
cavity of said article before said light passes out through said
perforations is restricted to a value x such that the output light energy
Es transmitted through said perforations is compatible with the light
energy detectable by said video camera, said output light energy Es being
linked to the energy Ee of the light emitted by said light source by the
relation Es=e.sup.x Ee, where e is the overall efficiency of transmission
as a function of the total number of reflections and of the reflectivity
coefficient of the material of the article, and x is the said number of
intermediate reflections.
5. A process according to claim 1, wherein said article is a hollow blade
for a turbine of a turbomachine, and said perforations are
micro-perforations in the vicinity of the leading or trailing edge of said
blade, said opening into said internal cavity of said blade being located
in the root of said blade.
6. Apparatus for carrying out a process for optically checking the
perforations of a hollow article having an internal cavity bounded by a
wall provided with a plurality of perforations therethrough, and an
opening into said cavity, said apparatus comprising:
a source of coherent light;
means for focussing the light emitted from said coherent light source;
means for holding the article to be checked so that the focussed light from
said light source enters said internal cavity of said article through said
opening;
a video camera for receiving light reflected from within said cavity
outwardly through said perforations;
a monitor screen connected to said video camera; and
an image processing module connected to said video camera,
said module comprising image storage units and units for processing
digitized signals corresponding to each image produced by said camera,
wherein said image processing module includes computing means for analysing
an luminance intensity curve of the light emitted by said perforations
sections by section of the displayed image of said perforations.
7. Apparatus according to claim 6, wherein said image processing module
includes computing means for processing said signals, by convolution and
filtering, to provide corrected images of a light signature of said
perforations.
8. Apparatus according to claim 6, for simultaneously checking a plurality
of said articles, wherein said source of light is a laser of p watts
power, and said apparatus comprises means for splitting the light beam
emitted by said laser source into a plurality of beams of P/n watts power
where n is the number of beams, focussing means for causing each of said
plurality of beams to illuminate a respective one of said articles to be
checked, and a separate video camera for each of said articles coupled to
said image processing module, said module being adapted to record and
process simultaneously the signals received from said video cameras.
9. Apparatus according to claim 8, wherein said beam splitting means
comprises a plurality of beam splitters of the plate or prism type
arranged in series.
10. Apparatus according to claim 8, wherein said beam splitting means
comprises a splitter box having a plurality of outputs constituted by
optical fibres, each fibre being adapted to convey a beam of P/n watts
power and having integrated optical focussing means at its end remote from
said splitter box.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process and apparatus for optically checking
perforations in hollow articles, and is particularly concerned with the
checking of micro-perforations in aircraft engine parts such as combustion
chamber walls, multi-perforated jackets of after-burner passages, and
hollow turbine blades.
In such parts it is essential to be able to check, or even measure, the
"permeability" of very small diameter holes. This permeability represents
the capacity of the holes (perforations) to allow an aeration or cooling
air flow through the wall in which they are drilled, and compliance with a
minimum permeability is generally vital to the life of the part concerned.
Thus, in one example of cooled blades for the first turbine stage of a
supersonic turbojet engine, in which the internal cavity of the blade is
supplied with pressurized air through its root, each blade has one row of
53 perforations along its leading edge and two rows of 80 and 19
perforations along the trailing edge, all of the perforations having an
average diameter of from 300 to 500 microns.
2. Summary of the prior art
The present method for checking the drilling quality of these holes
consists of performing manually two operations. Firstly a gauge rod is
inserted into each hole to check whether the hole is of a minimum diameter
equal to that of the rod and whether the hole opens out into the internal
cavity of the blade, and secondly the holes are counted to check that the
intended number of holes have been provided.
In addition to the fact that this method is lengthy and tedious, the risks
of error are great when counting the holes, and when checking the drilling
by means of gauge rods, some holes may be omitted if the operator's
attention is distracted. Moreover, these checks do not indicate whether
the holes have been drilled in the right place, or whether the drilling
accuracy (non-circular, etc.) is correct.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an optical
method of checking the perforations of perforated hollow articles,
particularly hollow turbine blades as described above, which method avoids
all the above drawbacks and makes it possible to ensure rapid checking of
the quality of the perforations, their number and the accuracy of their
siting, with as little human intervention as possible.
A further object of the present invention is to provide practical apparatus
for implementing the checking method which enables articles to be checked
either individually or simultaneously.
According to the invention there is provided a process for optically
checking the perforations of a hollow article having an internal cavity
bounded by a wall provided with a plurality of perforations therethrough,
and an opening into said cavity, comprising the steps of:
using a light source to illuminate the internal cavity of said article
through said opening;
scanning a video camera along the outside of said article to receive light
reflected from within said internal cavity outwardly through said
perforations and thereby collect luminance data relating to said reflected
light transmitted through said perforations;
converting the sequence of said collected luminance data into electrical
signals;
storing said signals in a computing and storage unit; and
processing said signals for comparison with a predetermined train of
reference signals derived from a standard article.
The processing of the measurement signals may include a visual analysis of
an image of the article on a video screen, with or without comparison on
the screen with a reference article.
The processing may also include a step of analysing the intensity of the
luminance of said light received by said camera, said analysis step being
performed section by section of the video image provided by said camera.
Further according to the invention there is provided apparatus for carrying
out a process for optically checking the perforations of a hollow article
having an internal cavity bounded by a wall provided with a plurality of
perforations therethrough, and an opening into said cavity, said apparatus
comprising:
a source of coherent light;
means for focussing the light emitted from said coherent light source;
means for holding the article to be checked so that the focussed light from
said light source enters said internal cavity of said article through said
opening;
a video camera for receiving light reflected from within said cavity
outwardly through said perforations;
a monitor screen connected to said video camera; and
an image processing module connected to said video camera,
said module comprising image storage units and units for processing
digitized signals corresponding to each image produced by said camera.
In the case where it is desired to check a plurality of articles
simultaneously using a single source of coherent light, such as a laser of
p watts power, the apparatus comprises means for splitting the light beam
emitted by said laser source into a plurality of beams of P/n watts power
where n is the number of beams, focussing means for causing each of said
plurality of beams to illuminate a respective one of said articles to be
checked, and a separate video camera for each of said articles coupled to
said image processing module, said module being adapted to record and
process simultaneously the signals received from said video cameras.
Other characteristics of the checking process and apparatus in accordance
with the invention will become apparent from the following description of
the preferred embodiments with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram showing the principle of the checking
apparatus in accordance with the invention applied to the checking of a
hollow turbine blade;
FIG. 2 is a schematic diagram of an embodiment of the apparatus for the
simultaneous checking of several blades;
FIG. 3 is a schematic diagram of an alternative embodiment for
simultaneously checking a plurality of blades;
FIG. 4 shows a photograph of an example of a model X cooled turbine blade,
the leading edge of which has one row of 53 holes, and the trailing edge
of which has two rows of 80 and 19 holes for the outflow of air;
FIG. 5 shows a photograph of an optical image of the trailing edge of the
blade shown in FIG. 4, obtained by means of a process in accordance with
the invention;
FIG. 6 shows a photograph of a model Y blade which has its leading edge
provided with three rows of 15 air outflow holes;
FIG. 7 shows a digitized image of the leading edge of the Y blade, viewed
from the extrados side, and also the use of luminance curves for
interpreting the results read on the digitized image;
FIG. 8 shows the luminance curve corresponding to the section AA in FIG. 7;
FIG. 9 shows the perforation fault in the section AA of the blade in the
photograph of FIG. 7, as revealed by analysis;
FIG. 10a is a photograph of the model X blade showing the
micro-perforations of the trailing edge;
FIG. 10b is a photograph of the same blade showing the row of
micro-perforations of the leading edge;
FIGS. 11a to 11d show images reconstituted by digitizing and thresholding
the light signals derived from the openings of the trailing edge of the
blade shown in FIG. 10a, with FIG. 11a representing the right-hand part of
FIG. 10a and FIGS. 11b to 11d representing successive parts progressing
left therefrom towards the blade root.
FIGS. 12a to 12c show similar images of the leading edge seen in FIG. 10b,
FIG. 12a representing the part remote from the heel and FIG. 12c the part
adjacent the heel;
FIG. 13 is a reconstructed image of the trailing edge of a model Z blade
having two rows of 11 micro-perforations;
FIG. 14 is a photograph of the image of FIG. 13 processed to extract the
outline of each micro-perforation; and
FIGS. 15 and 15a to 15d show various stages of a method for the fine
calculation of skeleton images of the micro-perforations.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1 a section is shown of a hollow, air-cooled, model X blade 1
having a root 2 and a hollow aerofoil portion 3. In the example
illustrated the blade has an upstream channel 4 in the vicinity of its
leading edge which is supplied with cold air through the root 2, and the
air is discharged through 53 micro-perforations 5 made in the extrados in
the vicinity of the leading edge.
The blade also has a downstream cavity 6 supplied by two channels 7 in the
blade root 2 and having internal turbulence points or studs 8. The air is
discharged towards the trailing edge through two rows of
micro-perforations 9 of about 500 microns in diameter, each row having
respectively 80 and 19 perforations. The general principle of the
invention resides in illuminating such a hollow blade from the inside, and
examining and analyzing the radiated light which passes through the wall
of the blade through the openings 5 or 9 in order to measure the number
and characteristics of the openings.
To permit the implementation of this principle, the blade is illuminated by
a source of coherent light, which in the present example is a continuously
emitting laser tube 10 of the ionized argon type, of one watt power, which
produces a light beam of 514.4 nanometers wavelength.
The light energy Es available at the outlet of the micro-perforations is
equal to Es=e.sup.x Ee, where x is the number of reflections of the
radiation inside the cavity and Ee is the energy of the light emitted by
the light source.
This energy Es is received by a video camera 13, and it is necessary that
Es should be compatible with the energy levels detectable by such cameras,
e.g. a few milliwatts for a CCTV type low luminance level camera. The
image received by the camera 13 is transmitted to a video monitor 14 on
which it can be interpreted by an operator.
The illuminating beam 11 is focussed by means of a convergent lens 12 and
its optical axis is substantially oriented along the axis of symmetry of
the cavity to be illuminated, in this case the downstream cavity 6.
Depending on the internal structure of the blade being examined, the blade
is positioned relative to the laser source 10 so as to promote a
homogeneous distribution of light in the blade cavity. Furthermore, since
the focussed light beam encounters on its travel a certain number of
obstacles (i.e. the walls of the channels 7, studs 8, and the inner wall
of the cavity 6), from which it is reflected, before issuing through the
micro-perforations 5 or 9, it is important to limit the number of
reflections of the rays of light in the cavity because at each reflection
the intensity of reflected light Ir is equal to Ir=e Ie,
where
Ie is the intensity of incident light
e is the overall efficiency linked with the reflectivity coefficient of the
material.
For each reflection the following obtains:
##EQU1##
with n.sub.air and n.sub.metal being indices of refraction in air and
metal respectively and k being the absorption coefficient of the metal.
With respect to the materials used for making hollow blades the refraction
indices "n.sub.metal " of the said metals vary from 0.58 to 0.63 and the
absorption coefficient is nil. Thus, for a model X blade made from a
nickel-based alloy of the INCONEL 718 (trade name) type, comprising 19%
Cr, 18% Fe, 5% Nb, and the remainder Ni, the efficiency e is equal to
0.057 for one reflection.
For another alloy used for model Y blades, which will be referred to later,
i.e. a nickel-based alloy named DS 200 (trade name) comprising 12% W, 10%
Co, 9% Cr, 5% Al, and the remainder Ni, the efficiency e is equal to
0.076.
The signals emitted by the camera 13 are also transmitted to an
analogue/digital computer where they are digitized to be processed in an
image processing module 15. By convolution processing and filtering,
detection sensitivity is improved and the contrast between subsequent
perforations and the remainder of the part is increased. In this way
corrected images are recreated which it is possible to display on a screen
16 linked to the processing module 15. Moreover, as the image is stored,
it is possible for each scanning to establish the luminance curve relating
to the corresponding section of the part. The light intensity received
point by point and digitized is a function of the diameter of the
micro-perforation through which the corresponding ray of light issued.
Thus, it is possible to make a finer analysis of the exact condition of
the part.
Examples of operation will now be explained with reference to FIGS. 4 to 7.
Two types of hollow blades were examined by means of the apparatus in
accordance with the invention. The two types of blades were high pressure
turbine blades of a turboshaft engine, the first a model X blade made of
INCONEL 718, and the second a model Y blade made of DS 200. The number of
micro-perforations provided in the blades is summarised in the following
table:
______________________________________
Micro- Micro-perforations detected
perforations Video Corrected
existing image digitized image
leading trailing
leading
trailing
leading
trailing
PARTS edge edge edge edge edge edge
______________________________________
HP Blade
53 80 53 80
Model X +19 +19
HP Blade
15 14 14 14 15 14
Model Y +15 int. +15 15
+15 ext. +15 15
______________________________________
FIG. 4 is a photograph of the model X blade in which the trailing edge
perforations may be seen.
FIG. 5 is a photograph of the optical signature of the trailing edge
perforations such as may be seen on the video screen 14. It will be noted
that all of the perforations are plotted on the photograph of FIG. 4. The
two rows of perforations are visible and the number of perforations in
each row can be counted.
Perforations appearing darker than the others must be interpreted as having
a smaller diameter or as only partly clear.
FIG. 6 is a photograph of the leading edge of the model Y blade. In the
photograph of FIG. 7 the digitally reconstituted processed image shows two
of the three rows of 15 existing perforations. The orientation of the
third row of perforations does not make it visible in the photograph.
Whereas the video image (similar to that of FIG. 5) enabled only 14 holes
instead of the 15 existing holes to be counted in one of the two rows, the
reconstituted image of FIG. 7 made it possible, through improvement of the
contrast, to detect the 15 perforations expected.
The 15th hole, not visible in the video image, corresponds to section AA of
FIG. 7.
Analysis of the luminance curve of the section AA of the part containing
the majority of the perforations to be checked (chosen in a sectional
plane substantially perpendicular to a direction along the article
containing a majority of the perforations to be checked shows two peaks of
intensity. The first peak 17a, of greater height, corresponds to an
opening of normal diameter 18a, whereas the smaller second peak 17b
discloses the existence of an opening 18b which the video image did not
reveal. A more detailed analysis of the opening 18b showed that the latter
opened out obliquely and that the diameter "visible" to the camera was
0.15 mm instead of the 0.50 mm of a normal opening. Such a defect in
perforation geometry would not have been possible with the prior method of
checking using gauge rods.
The digitizing processes and their results will now be discussed with
reference to FIGS. 10 to 15d.
After digitization each peak of light intensity may be compared with a
predetermined binarization threshold, which permits a qualitative and
quantitative analysis in the counting of the open micro-perforations.
FIGS. 11a to 11d are the digitized images of the trailing edge photographed
in FIG. 10a. In FIG. 11a it may be observed that between the fourth and
fifth visible holes from the right of the top row, a dark gap longer than
the others indicates a hole which does not open out, and in the bottom row
the fifth hole from the right is observed to open out only slightly. This
qualitative analysis may be quantified and the geometric parameters of the
micro-perforations can be established.
Thus, if reference is made to FIG. 15, a portion of the display screen of
the image processing module 15 is shown, the screen being separated into
pixels of size "e". Line scanning makes it possible to attribute to each
pixel the binarized level of light intensity corresponding to a
perforation or a non-perforated part of the surface. By counting the
consecutive pixels illuminated it will be possible to ascertain the area
of each micro-perforation and to determine the center of intensity of each
micro-perforation, which is equated to the geometric center of the
perforation.
Having established the center of each micro-perforation it is possible to
deduce therefrom the distance separating consecutive center and thus the
distance between the holes.
It is also possible from this measurement to calculate the line of least
squares for each generatrix of micro-perforations and to determine the
dispersion of each perforation relative to this line of least squares.
A second method of processing permits refinement of the parameters just
discussed.
For each micro-perforation it is possible to calculate, pixel by pixel, the
mean value of the grey level of each pixel of a perforation, i.e. the
value I.sub.d of the intensity at the level of the pixel on abscissa d
related to the area e.sup.2 of said pixel, which makes it possible to
achieve an enhanced contrast of the image of each perforation.
Using this method, it is possible to optimize the measurement of the radius
of each perforation and to eliminate from the digitized image aberrant
points introduced into the measurement chain either by the optical system
or by the image system, this being achieved by eliminating from the
reconstituted image the points whose processed radius computed on the mean
value of grey levels would be lower than an imposed minimum value.
Similarly, using said "mean values of grey levels", it is possible to
recompute all the preceding data (distance between perforations,
dispersion of holes, their areas) in a finer manner.
If it is desired to make a quantitative analysis on an overall image of a
row of perforations, it will be possible to carry out an extraction of
outline for each perforation, i.e. to form a "skeleton image".
FIG. 13 shows the binarized image of a double row of perforations at the
trailing edge of a model Z blade, such as obtained by the processes
described above, whereas FIG. 14 shows the same as a "skeleton image"
obtained as follows.
As observed in FIGS. 7 and 8 the light intensity curve for each scanning is
disturbed by a noise factor. The same applies to the grey levels
calculated earlier. The aim of the outline extraction method is to
determine with extreme precision the edge of each micro-perforation.
To do this, at each scanning and for each perforation determining a grey
level peak, the grey level intensity curve I=f(d) is determined (FIG.
15a). To this function is added an interval function g=g(d) (FIG. 15b),
and a smoothing is made at p points of the obtained curve I'=f(d)+g(d),
the smoothed value
##EQU2##
being reallotted to the mean index point to provide a curve I"(d) which is
eroded (suppressing the ill-timed noise peaks), expanded (by the addition
of the function g(d)), and smoothed. From this curve the value .alpha. of
the tangent coefficient corresponding to the inflection point of the
smoothed curved I"(d) is extracted.
This operation is performed at each scanning and for each perforation, the
values thus established and stored permitting a reconstruction of the
precise outline of each perforation.
The photograph of FIG. 14 shows the reconstruction of the outline of each
perforation of FIG. 13 by this method, each point of each outline having
been determined by the method indicated above.
All parameters (area of perforations, spaces between perforations,
dispersion) can thus be recalculated with extreme precision.
The method of checking in accordance with the invention has just been
described in relation to the checking of a single article. However, the
method may be extended to the simultaneous checking of several articles.
For example, as shown in FIG. 2, (n-1) plate or prism beam splitters
19.sub.i in series with a mirror 20 may be interposed between the laser 10
and the n articles to be checked. The first splitter 19.sub.1 reflects a
ray of P/n power and transmits to the second splitter 19.sub.2 a power of
(n-1)/n P watts. The same applies to the second splitter 19.sub.2 which
transmits (n-2)/n P watts and so on up to the mirror 20 which receives and
reflects the last fraction P/n of the power from the source 10.
By means of mirrors 21 each split laser beam 11.sub.i is directed to the
respective article 1.sub.i to be checked, before which it is focussed by a
lens 12.sub.i.
For each article the reflection principle of operation is identical with
that described earlier, the apparatus comprising n cameras 13.sub.i in
parallel, and the image processing module 15 having n parallel stores and
one processing unit permitting the reconstruction of n corrected images of
the articles checked.
In an alternative arrangement shown in FIG. 3, the detection and computing
means are identical with those of the system of FIG. 2. The essential
difference lies in the laser beam transmission means used, the laser 10 of
P watts power emitting radiation into an optical splitter assembly 23
having n outputs constituted by optical fibres 23.sub.i each arranged at
its end with means 25 for focussing the beam on the article 1.sub.i to be
checked.
The apparatus in accordance with the invention may also be automated to
improve the checking rate by the addition of a gripping device for the
articles and a robot which positions the articles before each checking and
turns those parts whose leading edge and trailing edge perforations have
to be checked. Alternatively each article may remain stationary and a
robot used to carry the image collecting camera.
In addition, by permitting a comparison of the values of a checked article
with the reference values of a standard article stored in the computing
unit, it is possible to give a check verdict for each article pointing out
the conformity of the number of perforations, their diameter and the
distance between them.
Thus, the proposed method considerably improves checking quality and speed,
and it also permits checking perforations whose diameter may have
decreased down to 10 microns. Below this value a light beam parallel with
the inlet to the perforation to be checked would be diffracted as it
passes through, thus making the method impracticable.
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